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  26 <div class="section">
  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="boost_typeerasure.any"></a><a class="link" href="any.html" title="Using Any">Using Any</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="any.html#boost_typeerasure.any.construction">Construction</a></span></dt>
  32 <dt><span class="section"><a href="any.html#boost_typeerasure.any.conversions">Conversions</a></span></dt>
  33 <dt><span class="section"><a href="any.html#boost_typeerasure.any.references">References</a></span></dt>
  34 <dt><span class="section"><a href="any.html#boost_typeerasure.any.limit">Syntax Limitations</a></span></dt>
  35 </dl></div>
  36 <div class="section">
  37 <div class="titlepage"><div><div><h3 class="title">
  38 <a name="boost_typeerasure.any.construction"></a><a class="link" href="any.html#boost_typeerasure.any.construction" title="Construction">Construction</a>
  39 </h3></div></div></div>
  40 <p>
  41         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/construction.cpp" target="_top">construction.cpp</a>)
  42       </p>
  43 <p>
  44         The library provides the <code class="computeroutput"><a class="link" href="../boost/type_erasure/constructible.html" title="Struct template constructible">constructible</a></code>
  45         concept to allow an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
  46         to capture constructors. The single template argument should be a function
  47         signature. The return type must be a placeholder specifying the type to be
  48         constructed. The arguments are the arguments of the constructor.
  49       </p>
  50 <p>
  51 </p>
  52 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  53     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span>
  54     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_b</span><span class="special">&gt;,</span>
  55     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_c</span><span class="special">&gt;,</span>
  56     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">_b</span><span class="special">&amp;,</span> <span class="keyword">const</span> <span class="identifier">_c</span><span class="special">&amp;)&gt;</span>
  57 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
  58
  59 <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span>
  60     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
  61     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_b</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;,</span>
  62     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_c</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span>
  63 <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
  64
  65 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">&gt;</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">(</span><span class="number">10</span><span class="special">),</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
  66 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_c</span><span class="special">&gt;</span> <span class="identifier">val</span><span class="special">(</span><span class="number">2.5</span><span class="special">,</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
  67 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">v</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="identifier">val</span><span class="special">);</span>
  68 <span class="comment">// v holds std::vector&lt;double&gt;(10, 2.5);</span>
  69 </pre>
  70 <p>
  71       </p>
  72 <p>
  73         Now, suppose that we want a default constructor? We can't have the default
  74         constructor of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
  75         call the default constructor of the contained type, because it would have
  76         no way of knowing what the contained type is. So, we'll need to pass the
  77         placeholder binding information explicitly.
  78       </p>
  79 <p>
  80 </p>
  81 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  82     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
  83     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_self</span><span class="special">()&gt;</span>
  84 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
  85
  86 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
  87 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span> <span class="comment">// y == ""</span>
  88 </pre>
  89 <p>
  90       </p>
  91 <p>
  92         This method is not restricted to the default constructor. If the constructor
  93         takes arguments, they can be passed after the bindings.
  94       </p>
  95 <p>
  96 </p>
  97 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  98     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
  99     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_self</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)&gt;</span>
 100 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
 101
 102 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
 103 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">),</span> <span class="number">5</span><span class="special">,</span> <span class="char">'A'</span><span class="special">);</span>
 104 </pre>
 105 <p>
 106       </p>
 107 </div>
 108 <div class="section">
 109 <div class="titlepage"><div><div><h3 class="title">
 110 <a name="boost_typeerasure.any.conversions"></a><a class="link" href="any.html#boost_typeerasure.any.conversions" title="Conversions">Conversions</a>
 111 </h3></div></div></div>
 112 <p>
 113         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/convert.cpp" target="_top">convert.cpp</a>)
 114       </p>
 115 <p>
 116         An <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> can be converted
 117         to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> as long
 118         as the conversion is an "upcast."
 119       </p>
 120 <p>
 121 </p>
 122 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">any</span><span class="special">&lt;</span>
 123     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 124         <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 125         <span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span>
 126         <span class="identifier">ostreamable</span><span class="special">&lt;&gt;</span>
 127     <span class="special">&gt;</span>
 128 <span class="special">&gt;</span> <span class="identifier">any_printable</span><span class="special">;</span>
 129 <span class="keyword">typedef</span> <span class="identifier">any</span><span class="special">&lt;</span>
 130     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 131         <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 132         <span class="identifier">typeid_</span><span class="special">&lt;&gt;</span>
 133     <span class="special">&gt;</span>
 134 <span class="special">&gt;</span> <span class="identifier">common_any</span><span class="special">;</span>
 135 <span class="identifier">any_printable</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 136 <span class="identifier">common_any</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 137 </pre>
 138 <p>
 139       </p>
 140 <p>
 141         This conversion is okay because the requirements of <code class="computeroutput"><span class="identifier">common_any</span></code>
 142         are a subset of the requirements of <code class="computeroutput"><span class="identifier">any_printable</span></code>.
 143         Conversion in the other direction is illegal.
 144       </p>
 145 <p>
 146 </p>
 147 <pre class="programlisting"><span class="identifier">common_any</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 148 <span class="identifier">any_printable</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// error</span>
 149 </pre>
 150 <p>
 151       </p>
 152 </div>
 153 <div class="section">
 154 <div class="titlepage"><div><div><h3 class="title">
 155 <a name="boost_typeerasure.any.references"></a><a class="link" href="any.html#boost_typeerasure.any.references" title="References">References</a>
 156 </h3></div></div></div>
 157 <p>
 158         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/references.cpp" target="_top">references.cpp</a>)
 159       </p>
 160 <p>
 161         To capture by reference, we simply add a reference to the <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>.
 162       </p>
 163 <p>
 164 </p>
 165 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
 166 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
 167 <span class="identifier">any_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span> <span class="comment">// now i is 5</span>
 168 </pre>
 169 <p>
 170       </p>
 171 <div class="note"><table border="0" summary="Note">
 172 <tr>
 173 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 174 <th align="left">Note</th>
 175 </tr>
 176 <tr><td align="left" valign="top"><p>
 177           <code class="computeroutput"><span class="identifier">_self</span></code> is the default <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>, so it is
 178           easiest to use <code class="computeroutput"><span class="identifier">_self</span><span class="special">&amp;</span></code>.
 179           We could use another <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>
 180           instead. <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code><code class="computeroutput"><span class="special">&lt;</span></code><code class="computeroutput"><a class="link" href="../boost/type_erasure/typeid_.html" title="Struct template typeid_">typeid_</a></code><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span> <span class="identifier">_a</span><span class="special">&amp;&gt;</span></code> has exactly the same behavior.
 181         </p></td></tr>
 182 </table></div>
 183 <p>
 184         References cannot be rebound. Just like a built-in C++ reference, once you've
 185         initialized it you can't change it to point to something else.
 186 </p>
 187 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">;</span>
 188 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">j</span><span class="special">);</span>
 189 <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// error</span>
 190 </pre>
 191 <p>
 192       </p>
 193 <div class="note"><table border="0" summary="Note">
 194 <tr>
 195 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 196 <th align="left">Note</th>
 197 </tr>
 198 <tr><td align="left" valign="top"><p>
 199           As with any other operation, <code class="computeroutput"><span class="identifier">x</span>
 200           <span class="special">=</span> <span class="identifier">y</span></code>
 201           for references acts on <code class="computeroutput"><span class="identifier">i</span></code>
 202           and <code class="computeroutput"><span class="identifier">j</span></code>. Assignment like
 203           this is legal if <code class="computeroutput"><a class="link" href="../boost/type_erasure/assignable.html" title="Struct template assignable">assignable</a></code><code class="computeroutput"><span class="special">&lt;&gt;</span></code> is in the Concept, but <code class="computeroutput"><span class="identifier">x</span></code> would still hold a reference to <code class="computeroutput"><span class="identifier">i</span></code>.
 204         </p></td></tr>
 205 </table></div>
 206 <p>
 207         A reference can be bound to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
 208       </p>
 209 <p>
 210 </p>
 211 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 212     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 213     <span class="identifier">incrementable</span><span class="special">&lt;&gt;</span>
 214 <span class="special">&gt;</span> <span class="identifier">requirements</span><span class="special">;</span>
 215
 216 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 217 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 218 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// x is now 11</span>
 219 </pre>
 220 <p>
 221       </p>
 222 <p>
 223         If a reference is used after the underlying object goes out of scope or is
 224         reset, the behavior is undefined.
 225       </p>
 226 <p>
 227 </p>
 228 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 229     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 230     <span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span>
 231     <span class="identifier">relaxed</span>
 232 <span class="special">&gt;</span> <span class="identifier">requirements</span><span class="special">;</span>
 233 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 234 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 235 <span class="identifier">x</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
 236 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// undefined behavior.</span>
 237 </pre>
 238 <p>
 239       </p>
 240 <p>
 241         This only applies when a reference is constructed from a value. If a reference
 242         is constructed from another reference, the new reference does not depend
 243         on the old one.
 244       </p>
 245 <p>
 246 </p>
 247 <pre class="programlisting"><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 248 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">p</span><span class="special">(</span>
 249     <span class="keyword">new</span> <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">));</span>
 250 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(*</span><span class="identifier">p</span><span class="special">);</span> <span class="comment">// equivalent to y(x);</span>
 251 <span class="identifier">p</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span>
 252 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// okay</span>
 253 </pre>
 254 <p>
 255       </p>
 256 <p>
 257         Both const and non-const references are supported.
 258       </p>
 259 <p>
 260 </p>
 261 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
 262 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
 263 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 264 </pre>
 265 <p>
 266       </p>
 267 <p>
 268         A reference to non-const can be converted to a reference to const, but not
 269         the other way around. Naturally, we can't apply mutating operations to a
 270         const reference.
 271       </p>
 272 <pre class="programlisting"><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">z</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// error</span>
 273 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// error</span>
 274 </pre>
 275 </div>
 276 <div class="section">
 277 <div class="titlepage"><div><div><h3 class="title">
 278 <a name="boost_typeerasure.any.limit"></a><a class="link" href="any.html#boost_typeerasure.any.limit" title="Syntax Limitations">Syntax Limitations</a>
 279 </h3></div></div></div>
 280 <p>
 281         In most cases using an any has the same syntax as using the underlying object.
 282         However, there are a few cases where this is not possible to implement. An
 283         <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> reference is proxy
 284         and cannot be used in contexts where a real reference is required. In particular,
 285         <code class="computeroutput"><a class="link" href="../boost/type_erasure/forward_iterator.html" title="Struct template forward_iterator">forward_iterator</a></code>
 286         does not create a conforming ForwardIterator (unless the value_type is fixed.)
 287         Another difference is that all operations which do not take at least one
 288         <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> argument have to
 289         be passed the type information explicitly. Static member functions and constructors
 290         can fall in this category. All this means that generic algorithms might not
 291         work when applied to <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
 292         arguments.
 293       </p>
 294 </div>
 295 </div>
 296 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 297 <td align="left"></td>
 298 <td align="right"><div class="copyright-footer">Copyright &#169; 2011-2013 Steven Watanabe<p>
 299         Distributed under the Boost Software License, Version 1.0. (See accompanying
 300         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 301       </p>
 302 </div></td>
 303 </tr></table>
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